Ultrahigh critical current densities, the vortex phase diagram, and the effect of granularity of the stoichiometric high- Tc superconductor CaKFe4As4

Abstract

We present a comprehensive study of the critical current densities and the superconducting vortex phase diagram in the stoichiometric superconductor CaKFe4As4 which has a critical temperature of ∼ 35 K. We performed detailed magnetization measurements both of high quality single crystals for different orientations in an applied magnetic field up to 16 T and for a powder sample. We find an extremely large critical current density, Jc, up to 10 8 A/cm 2 for single crystals when H||(ab) at 5 K, which remains robust in fields up to 16 T, being the largest of any other iron-based superconductor. The critical current density is reduced by a factor 10 in single crystals when H||c at 5 K and significantly suppressed by the presence of grain boundaries in the powder sample. We also observe the presence of the fishtail effect in the magnetic hysteresis loops of single crystals when H||c. The flux pinning force density and the pinning parameters suggest that the large critical current could be linked to the existence of point core and surface pinning. Based on the vortex phase diagram and the large critical current densities, CaKFe4As4 is now established as a potential iron-based superconductor candidate for practical applications. arXiv:1808.06072v1 [cond-mat.supr-con]

Competing pairing interactions responsible for the large upper critical field in a stoichiometric iron-based superconductor CaKFe4As4

“…This supports the extremely large critical current densities, reported previously for CaKFe 4 As 4 in Ref. [14]. The small coherence length of CaKFe 4 As 4 is compatible with the presence of the large upper critical field, consistent with small Fermi velocities and high T c values of CaKFe 4 As 4 (ξ ∼ v F /(2πk B T c ) [1]).…”

“…Previous magnetisation reported µ 0 H c1 = 22(1) mT [14] when H||c at low temperature which, combined with H c2 values reported here, give κ ∼ 99(2) and λ = 183(6) nm. Using ξ = 1.86 nm, we find that the depairing current density is J d ∼ 1.62 × 10 8 A/cm 2 , which is among the largest of any known iron-based superconductor.…”

“…This system lacks long-ranged magnetic order or a nematic electronic state at low temperatures [5][6][7][8][9][10][11] but upon doping with Ni a hedgehog magnetic structure is stabilized [9,12,13]. CaKFe 4 As 4 has an exceptionally large critical current density due to the strong point-like defects caused by local structural site effects as well as surface pinning [14][15][16]. Due to reduced symmetry compared with the 122 family of iron-based superconductors, CaKFe 4 As 4 is predicted to have up to ten different bands ( Fig.…”

Unique defect structure and advantageous vortex pinning properties in superconducting CaKFe4As4

“…Figure 3b shows the H dependence of Jc H//ab derived from the MHLs. Here, we applied a simplified calculation procedure following the previous work 30 Figure 3c shows the MHLs for BaK122 (x = 0.4). In contrast to the case of CaK1144, the MHLs show a peak around self-field, similar to that for H // c. Figure 3d shows the H dependence of Jc H//ab for BaK122.…”

“…In fact, recent studies on vortex pinning properties of CaK1144 reported unusually high Jc 30 as well as vortex dynamics distinct from 122 materials, 31 while the relevant pinning mechanisms remain unsolved. This motivated us to explore the microstructure and the vortex pinning mechanisms in CaK1144.…”